Para-xylene is in demand as a precursor in the production of terephthalic acid and dimethyl terephthalate which are, in turn, used in the manufacture of polyester resins. Mixtures containing substantial guantities of ethylbenzene and xylene isomers are prepared by catalytic reforming of petroleum fractions and catalytic isomerization of xylenes. Typical processes for separating p-xylene from C.sub.8 aromatic mixtures involve fractional crystallization. In order to provide an improved feedstock for para-xylene crystallization processes it is desirable to reduce the concentration of ethylbenzene in the aromatic mixture. Ethylbenzene acts as a diluent in crystallization minimizing the production of p-xylene from a given size crystallization apparatus and lowering the eutectic temperature of the mixtue which, if obtained, results in a higher viscosity slurry and a lower density difference between the p-xylene crystals and the mother liquor. Thus, the presence of ethylbenzene impedes the separation of p-xylene crystals.
It is known that the boiling points of the C.sub.8 aromatics are very close. In particular, ethylbenzene boils at about 136.degree. C., p-xylene at about 138.degree. C., m-xylene at about 139.degree. C., and o-xylene at about 144.degree. C. While ethylbenzene can be separated from xylene isomers by fractional distillation, typical ethylbenzene fractionators contain 300 to 400 actual trays and require about a 25-50 to 1 feed ratio.
Accordingly, there is a continuing need to provide a process for reducing the concentration of ethylbenzene in C.sub.8 aromatic mixtures, thereby providing a more efficient less expensive p-xylene crystallization feedstock. One approach to the separation of ethylbenzene and xylenes has been to hydrogenate the C.sub.8 mixture prior to separation. Interest in this approach stems from the greater difference in boiling points between ethylcyclohexane and the dimethyl cyclohexanes than between ethylbenzene and the xylenes. U.S. Pat. No. 2,282,231 describes such a process. While complete hydrogenation of the C.sub.8 mixture is satisfactory, it suffers certain shortcomings. For example, it is suggested that the feedstock should not contain o-xylene. Thus, the C.sub.8 mixture should first be distilled to separate the o-xylene fraction. It is also necessary to reform the resulting substituted cyclohexanes to xylenes in order to obtain the desired p-xylene-containing mixture. Thus, an additional step is required. These shortcomings could be overcome by providing a hydrogenation process which selectively hydrogenates ethylbenzene in a C.sub.8 aromatic mixture.